1. Field of the Invention
The present invention relates to methods and systems for processing one or more microstructures of a multi-material device.
2. Background Art
Conventionally, round spots have been used for a majority of precision-scanned, laser-processing applications. Many laser sources such as Nd:YAG lasers produce round Gaussian beams, which, when imaged through conventional spherical optics, produce round spots. These spots are scanned across target sites to process material, and the resulting laser-material interaction removes or otherwise alters the targeted material. In many laser-processing applications, system throughput is limited by the average power, the substrates"" fluence damage threshold, and, in pulsed systems, the laser q-rate and the laser pulse characteristics.
Exemplary micromachining operations include link blowing of redundant memory circuits, laser trimming, and circuit fabrication. For processing applications such as blowing sub-micron width fuses on a memory device, efficient coupling of energy to a narrow fuse with minimum lateral and substrate damage is desirable. Large round spots may cause undesirable adjacent link damage as shown in FIG. 19a. While smaller spots allow finer pitched fuses to be processed, the potential for substrate damage can increase with the higher fluence of a decreasing spot size as shown in FIG. 19b. When micromachining a line made from a sequence of small laser spots, the spot overlap or so-called xe2x80x9cbite sizexe2x80x9d and q-rate are two of the process characteristics that determine maximum scan velocity. Laser trimming applications requiring a wide kerf width may require multiple passes with a small spot when the peak fluence of a larger spot is inadequate. In the field of a lead frame fabrication, wherein a fine pitch lead on a large lead count device is machined, a rotating elongated spot is used as described in U.S. Pat. No. 5,632,083.
Out-of-round spots are often considered as system defects that limit process quality. Much effort has been expended in the field of laser optics to improve beam quality, to circularize beams from diode lasers, and to design and implement highly corrected optics for diffraction-limited systems. Vector diffraction effects used for beam-shape compensation are described in U.S. Pat. No. 4,397,527.
Many techniques are known for beam shaping and spot shaping. One method is a phase plate used with a round beam to modify the spot shape for processing memory fuses as shown in the upper portion of FIG. 20 and as taught by Cordingley in U.S. Pat. No. 5,300,756. The primary effect using this simple type of phase plate is to create a top-hat distribution profile as shown in the lower portion of FIG. 20. However, techniques for creating an oblong spot are also described.
Use of an anamorphic spot with dithering for shaping a laser beam intensity profile is described in U.S. Pat. No. 6,341,029. The anamorphic spot allows sharper line edges to be formed with a narrowed spot width, while an increased spot length maintains desired total power without exceeding process limits on integrated power per unit substrate area.
Veldkamp in U.S. Pat. No. 4,410,237 describes a diffraction grating and prism method for transforming a round Gaussian beam to elongated flat-top profiles.
Dickey in U.S. Pat. No. 5,864,430 describes a phase-based method for transforming a round Gaussian beam to a flat-top, square, or rectangular-shaped spot.
Yet another technique is creating an array of spots such as disclosed by James in U.S. Pat. No. 5,463,200.
Another well known technique is the imaged aperture mask.
Apodization is yet another simple technique to modify the beam shape and thereby spot shape.
Published U.S. patent application in the name of Baird et al., U.S. 2002/0005396 A1, discloses a UV laser system wherein an optics module is provided to enhance shape quality of laser beams.
Sun et al. in U.S. Pat. No. 5,265,114 describe a method and system for selectively laser processing a target structure of one or more materials of a multi-material, multi-layer device.
Sun et al. in U.S. Pat. No. 6,057,180 describe a method for severing electrically conductive links with ultraviolet laser output.
An object of the present invention is to provide an improved method and system for processing one or more microstructures of a multi-material device.
In carrying out the above object and other objects of the present invention, a method for processing at least one microstructure which is part of a multi-material device containing a plurality of microstructures is provided. The at least one microstructure has a designated region for target material removal. The method includes generating a laser beam, modifying the laser beam to obtain a modified laser beam, and sequentially and relatively positioning the modified laser beam into at least one non-round spot having a predetermined non-round energy distribution on the designated region to remove the target material in the designated region. The predetermined non-round energy distribution covers an area of the designated region such that energy is more efficiently coupled into the designated region for the non-round energy distribution than energy coupled into the designated region for a round energy distribution covering the same area.
The predetermined non-round energy distribution may include pre-specified characteristics including an aspect ratio, a focused spot size, an orientation, depth of focus and a focused irradiance distribution.
The at least one microstructure may be a link structure having a length and the multi-material device is a semiconductor device. The designated region may be located between but does not include electric contacts for the link structure.
The designated region may be less than 80% of the length of the link structure between the contacts.
The at least one non-round spot may have a major axis aligned with the length of the link structure.
The at least one non-round spot may have an aspect ratio greater than about 1.2.
The at least one non-round spot may further have an aspect ratio greater than about 1.2 and less than about 80% of the length of the link structure.
The at least one non-round spot may still further have an aspect ratio greater than 1.5.
The at least one microstructure may have a rectangular shape with a dimension less than 1 xcexcm in a narrow dimension of the rectangular shape.
The at least one microstructure may further have a rectangular shape with a dimension less than 0.8 xcexcm in a narrow dimension of the rectangular shape.
The at least one microstructure may still further have a rectangular shape with a dimension less than 0.5 xcexcm in a narrow dimension of the rectangular shape.
The at least one microstructure may have an aspect ratio of at least 4:1 in the designated region.
The microstructures may be located on a semiconductor substrate of the device.
The non-round energy distribution may be an elliptical Gaussian.
The non-round energy distribution may further be a top hat in a first dimension and a Gaussian in a second dimension substantially orthogonal to the first dimension.
The first dimension may be along a length of the at least one microstructure.
The step of positioning may be repeated to process a plurality of microstructures within a field with a plurality of non-round spots having a corresponding plurality of predetermined non-round energy distributions.
Each non-round spot has an orientation and each microstructure has an orientation. The step of positioning may include aligning the orientations of the non-round spots to corresponding orientations of the microstructures.
The orientations of the plurality of processed microstructures may be orthogonal orientations.
The step of aligning may be controlled automatically based on predetermined microstructure orientations.
The predetermined microstructure orientations may be contained in a wafer repair file.
The processed microstructures may be metal links of a multi-material, redundant memory device.
The step of positioning may include the step of aligning an axis of the at least one non-round spot with the at least one microstructure.
The step of aligning may be performed automatically, and the step of aligning may include switching the laser beam to one of a plurality of optical paths.
The laser beam may be polarized, and the step of switching may include controllably modifying the polarization of the laser beam.
The step of switching may further include controllably modifying the laser beam with an anamorphic optical system.
The step of aligning may include at least semi-automatically adjusting a major axis of the at least one non-round spot.
The step of aligning may further include providing computer-generated signals to automatically adjust a major axis of the at least one non-round spot.
The step of aligning may still further include automatically moving an optical subsystem in response to orientation control signals.
The step of moving the optical subsystem may include moving an anamorphic optical component of the subsystem.
The microstructures contained in the device may be regularly arranged in rows and columns.
The predetermined non-round energy distribution may be based on a model of radiation-material interaction correlating a cross section of the designated region with shape of the at least one non-round spot.
The model may be a thermal model, or may be a multi-parameter model.
The step of positioning may at least partially be performed with a low inertia beam deflector.
The step of positioning may be at least partially performed with a movable translation stage.
The step of modifying may include the step of controllably modifying an aspect ratio of the laser beam with an anamorphic optical element.
The step of controllably modifying may include generating a control signal and adjusting an anamorphic optical system to adjust the aspect ratio in response to the control signal.
The at least one non-round spot may have a minor diameter, and the non-round energy distribution increases peak fluence at the designated region more slowly compared to peak fluence of a decreasing round spot with a similar minor diameter.
The at least one non-round spot may have a minor diameter, and positioning sensitivity of the at least one non-round spot is less than positioning sensitivity of a round spot with a similar minor diameter.
Peak fluence at the designated region may be reduced but energy coupled into the designated region is not reduced.
The target material in the designated region may be cleanly removed.
The target material in the designated region may be removed without undesirable material change to adjacent microstructures of the device.
The target material in the designated region may further be removed without undesirable material change to underlying layers of the device.
The target material in the designated region may still further be removed without undesirable material change to a substrate of the device.
The non-round energy distribution may have an edge profile parallel to an edge of the at least one microstructure.
The method may further include the step of increasing maximum energy of the at least one non-round spot.
The method may further include the step of decreasing minimum energy of the at least one non-round spot.
Further in carrying out the above objects and other objects of the present invention, a system for processing at least one microstructure which is part of a multi-material device containing a plurality of microstructures is provided. The at least one microstructure has a designated region for target material removal. The system includes means for generating a laser beam, and means for modifying the laser beam to obtain a modified laser beam. The system also includes means for sequentially and relatively positioning the modified laser beam into at least one non-round spot having a predetermined non-round energy distribution on the designated region to remove the target material in the designated region. The predetermined non-round energy distribution covers an area of the designated region such that energy is more efficiently coupled into the designated region for the non-round energy distribution than energy coupled into the designated region for a round energy distribution covering the same area.
The predetermined non-round energy distribution may include pre-specified characteristics including an aspect ratio, a focused spot size, an orientation, depth of focus and a focused irradiance distribution.
The at least one microstructure may be a link structure having a length and the multi-material device is a semiconductor device. The designated region may be located between but does not include electric contacts for the link structure.
The predetermined non-round energy distribution may be based on a model of radiation-material interaction correlating a cross section of the designated region with shape of the at least one non-round spot.
The means for modifying may include an anamorphic optical element for controllably modifying an aspect ratio of the laser beam.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.